The A2A Simulations Community

I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?

Medtner wrote:I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?

Lowering RPM literally means the pistons are sucking in less air, thus less vacuum past the MP probe. Should result in a rise in MP.
DH

DHenriquesA2A wrote:

Medtner wrote:I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?

Lowering RPM literally means the pistons are sucking in less air, thus less vacuum past the MP probe. Should result in a rise in MP.
DH

This is what I thought - and this is correctly modelled in the 182 (and in the other warbirds?). That means that there is a bug there somehow?

Medtner wrote:

DHenriquesA2A wrote:

Medtner wrote:I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?

Lowering RPM literally means the pistons are sucking in less air, thus less vacuum past the MP probe. Should result in a rise in MP.
DH

This is what I thought - and this is correctly modelled in the 182 (and in the other warbirds?). That means that there is a bug there somehow?

Could be. I'll pass it on to Scott for possible change in an update. Thanks. Good catch ! Our forum community in action )
DH

DHenriquesA2A wrote: Could be. I'll pass it on to Scott for possible change in an update. Thanks. Good catch ! Our forum community in action )
DH

Thanks to you too!

(btw I can't find words on how much it tickles me that the T6 is in development now! New blood to the warbird-family! )

Hi Dudley and Medtner,

I've noticed this behavior in the P-47 as well, and just confirmed it during a flight. I reduce MP to 32" then brought the propeller back to 2150 RPM which resulted in a drop of MP to around 30" if not just a tick below. Could you forward this on as well if it is considered abnormal?

JP

Jacques wrote:Hi Dudley and Medtner,

I've noticed this behavior in the P-47 as well, and just confirmed it during a flight. I reduce MP to 32" then brought the propeller back to 2150 RPM which resulted in a drop of MP to around 30" if not just a tick below. Could you forward this on as well if it is considered abnormal?

JP

No problem.

Mustang is the same way. I thought manifold pressure was dropping because the engine is turning the supercharger slower at a lower RPM?

AviationAtWar wrote:Mustang is the same way. I thought manifold pressure was dropping because the engine is turning the supercharger slower at a lower RPM?

Correct. Increasing the rpm of a radial compressor directly increases the pressure ratio and therefore the mp (if no mp regulation device is used). For the higher mass flows however at the extreme right choking border of the compressor map, there won't be any rise possible in pressure ratio any more.

With the supercharged engines you can only see the MP rise at near idle RPMs and low MP. I think, this is correct in the P-40.

I can't argue against better knowledge - I don't know that much about this, but based on what little I know and what I have seen it still seems weird.

Have a look at the following videos, with Kermit Weeks doing runups and propeller-exercises. With some careful attention one clearly sees him setting MP to the current air pressure: 30 inches in both cases, and that it rises to 33-34 ish when he cycles the prop.

AT6:

https://youtu.be/gAHNnHBBJT8?t=755

P-51
https://youtu.be/J1F_UJaaP1A?t=334

Edit:

https://www.youtube.com/watch?v=eOXxUApaaWo
In this part with the P-51 one can see that the MP stays put or (maybe - not completely clear) even rises ever so slightly when Kermit lowers the RPM after take off.

Listen to the wonderful engine, by the way! Goosebump-material!

P-51 has a manifold pressure regulator, so you set your desired MP. If anything changes (e.g. engine RPM), but the engine can still produce your desired MP, the regulator will compensate for the changes in MP which are caused by changing RPM.
On the other engines, you directly control the throttle plate.

Interesting here: http://www.avweb.com/news/pelican/Pelic ... 081-1.html There are some others, describing turbo are supercharger behaviour I htink.